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Register a new userHigh resolution Hall measurements across the VO2 metal-insulator transition reveal impact of spatial phase separation
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Many strongly correlated transition metal oxides exhibit a metal-insulator transition (MIT), the manipulation of which is essential for their application as active device elements. However, such manipulation is hindered by lack of microscopic understanding of mechanisms involved in these transitions. A prototypical example is VO2, where previous studies indicated that the MIT resistance change correlate with changes in carrier density and mobility. We studied the MIT using Hall measurements with unprecedented resolution and accuracy, simultaneously with resistance measurements. Contrast to prior reports, we find that the MIT is not correlated with a change in mobility, but rather, is a macroscopic manifestation of the spatial phase separation which accompanies the MIT. Our results demonstrate that, surprisingly, properties of the nano-scale spatially-separated metallic and semiconducting domains actually retain their bulk properties. This study highlights the importance of taking into account local fluctuations and correlations when interpreting transport measurements in highly correlated systems.
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Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly strong dependence on strain that is not predicted by band theory, emphasizing the relevance of the O ion to the physics of VO2.
First-order phase transitions in solids are notoriously challenging to study. The combination of change in unit cell shape, long range of elastic distortion, and flow of latent heat leads to large energy barriers resulting in domain structure, hysteresis, and cracking. The situation is still worse near a triple point where more than two phases are involved. The famous metal-insulator transition (MIT) in vanadium dioxide, a popular candidate for ultrafast optical and electrical switching applications, is a case in point. Even though VO2 is one of the simplest strongly correlated materials, experimental difficulties posed by the first-order nature of the MIT as well as the involvement of at least two competing insulating phases have led to persistent controversy about its nature. Here, we show that studying single-crystal VO2 nanobeams in a purpose-built nanomechanical strain apparatus allows investigation of this prototypical phase transition with unprecedented control and precision. Our results include the striking finding that the triple point of the metallic and two insulating phases is at the transition temperature, T_tr = T_c, which we determine to be 65.0 +- 0.1 C. The findings have profound implications for the mechanism of the MIT in VO2, but in addition they demonstrate the importance of such an approach for mastering phase transitions in many other strongly correlated materials, such as manganites and iron-based superconductors.
The metal-insulator transition (MIT) of VO2 is discussed with particular emphasis on the structural instability of the rutile compounds toward dimerization. Ti substitution experiments reveal that the MIT is robust up to 20% Ti substitutions and occurs even in extremely thin V-rich lamellas in spinodally decomposed TiO2-VO2 composites, indicating that the MIT is insensitive to hole doping and essentially takes on a local character. These observations suggest that either electron correlation in the Mott-Hubbard sense or Peierls (Fermi-surface) instability plays a minor role on the MIT. Through a broad perspective of crystal chemistry on the rutile-related compounds, it is noted that VO2 and another MIT compound NbO2 in the family eventually lie just near the borderline between the two structural groups with the regular rutile structure and the distorted structures characterized by the formation of dimers with direct metal-metal bonding. The MITs of VO2 and NbO2 are natural consequences of structural transitions between the two groups, as all the d electrons are trapped in the bonding molecular orbitals of dimers at low temperatures. Such dimer crystals are ubiquitously found in early transition metal compounds having chain-like structures, such as MoBr3, NbCl4, Ti4O7, and V4O7, the latter two of which also exhibit MITs probably of the same origin. In a broader sense, the dimer crystal is a kind of molecular orbital crystals in which virtual molecules made of transition metal atoms with partially-filled t2g shells, such as dimers, trimers or larger ones, are generated by metal-metal bonding and are embedded into edge- or face-sharing octahedron networks of various kinds. The molecular orbital crystallization opens a natural route to stabilization of unpaired t2g electrons in crystals.
Nucleation processes of mixed-phase states are an intrinsic characteristic of first-order phase transitions, typically related to local symmetry breaking. Direct observation of emerging mixed-phase regions in materials showing a first-order metal-insulator transition (MIT) offers unique opportunities to uncover their driving mechanism. Using photoemission electron microscopy, we image the nanoscale formation and growth of insulating domains across the temperature-driven MIT in NdNiO3 epitaxial thin films. Heteroepitaxy is found to strongly determine the nanoscale nature of the phase transition, inducing preferential formation of striped domains along the terraces of atomically flat stepped surfaces. We show that the distribution of transition temperatures is an intrinsic local property, set by surface morphology and stable across multiple temperature cycles. Our data provides new insights into the MIT of heteroepitaxial nickelates and points to a rich, nanoscale phenomenology in this strongly correlated material.
VO2 is a strongly correlated material, which undergoes a reversible metal insulator transition (MIT) coupled to a structural phase transition upon heating (T= 67{deg} C). Since its discovery the nature of the insulating state has long been debated and different solid-state mechanisms have been proposed to explain its nature: Mott-Hubbard correlation, Peierls distortion or a combination of both. Moreover, still now there is a lack of consensus on the interplay between the different degrees of freedom: charge, lattice, orbital and how they contribute to the MIT. In this manuscript we will investigate across the MIT the orbital evolution induced by a tensile strain applied to thin VO2 films. The strained films allowed to study the interplay between orbital and lattice degrees of freedom and to clarify MIT properties.
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